Display file extension

In my template file i’ve got a custom file upload field. I want it to display for example, ‘Download PDF’ where PDF is the actual file extension.

It won’t just be PDF files so i can’t just put that. I need some php that is going to show the file extension.

I’ve found this but i’m unsure how to use it.

http://codex.wordpress.org/Function_Reference/wp_check_filetype

Thanks